I have 2 questions about tennis
a) How come players take 3 balls, choose 2, and return one to the ball-boy? Is it, as I think, to be used specifically in case of a fault, or is there a greater purpose?
b) There are line referees for the 3 vertical lines, but does the umpire call the faults for the 4 horizontal ones, or are there more line referees for this.
Thanks in advance!
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